1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and particularly, to a control system for an internal combustion engine having a turbocharger and an exhaust gas recirculation mechanism.
2. Description of the Related Art
A variable displacement turbocharger having a compressor wheel for pressurizing the intake air of an internal combustion engine, a turbine wheel connected to the compressor wheel, and movable vanes for changing a flow rate of exhaust gases that are injected toward the turbine wheel, is widely known. The turbine wheel is rotationally driven by the kinetic energy of the exhaust gases.
An internal combustion engine having an exhaust gas recirculation mechanism for recirculating exhaust gases to the intake system is also widely known and is used to reduce the amount of NOx contained in the exhaust gases. The exhaust gas recirculation mechanism has an exhaust gas recirculation control valve that controls the amount of exhaust gas that is recirculated. A change in the opening of the exhaust gas recirculation control valve affects the pressure supplied to the turbine wheel (exhaust pressure on the upstream side of the turbine wheel) and the boost pressure.
Japanese Patent Laid-open No. JP2003-529716 (JP '716) discloses a control system which controls the opening of the exhaust gas recirculation control valve and the movable vanes of the turbocharger by using a combustion model of the internal combustion engine. According to the control system described by JP '716, a change in the intake air flow rate, which depends on a control amount needed for controlling an opening of the exhaust gas recirculation control valve, is input into a boost pressure control block, whereby an opening of the movable vane is adjusted according to the amount of recirculated exhaust gas.
In the control system disclosed by JP '716, the opening of movable vanes (vane opening) is basically set according to the demand torque of the engine and the engine rotational speed. Also, the set opening is corrected based on a deviation of the actual intake air amount from the demand intake air amount. In the correction control of the vane opening, the change in the intake air flow rate depending on the amount of recirculated exhaust gas is taken into consideration. In other words, according to the control system of JP '716, the parameter indicative of the exhaust condition in the exhaust pipe is not used for controlling the vane opening. Instead, the vane opening is merely limited by a limiter. Therefore, there remains a need for improvement in controlling the exhaust pressure, the boost pressure, and the recirculation amount of exhaust gases that are relevant to each other.